1. Field of the Invention
The present invention relates to a curing agent mainly composed of a hexamethylene diisocyanate type polyisocyanate compound which contains a uretidine dione dimer and an isocyanurate cyclic trimer. The present invention also relates to a coating composition and an adhesive composition employing the above curing agent.
2. Related Background Art
Coating materials and adhesives have come to be strongly desired to meet the requirements on material saving, low environmental pollution, and safety. Consequently, coating materials and adhesives of high solid type, powder type, and aqueous type, and adhesives of hot-melt type are come to be high-lighted. Particularly in the United States, high-solid types of coating materials and adhesives are being used to meet the regulation of VOC (volatile organic compounds). In Japan, although the conversion to the high-solid type is delayed in comparison with the conversion in the United States at the moment, the use of the high-solid type of coating materials and adhesives is being investigated because of the movement in environment protection in local governments, and present status of improvement of the conventional coating materials with conversion to high-solid type, and further the trend of unification of quality guarantee system of U.S.-made cars and Japanese-made cars in automobile industries, and so forth.
On the other hand, in practical coating operation, the coating materials needs to be diluted with a suitable solvent to a viscosity required for an coating method to be applied. When a high-viscosity varnish is used in the coating, a larger amount of the solvent has to be used for thinning, which decreases the coating thickness obtained by one coating operation and necessitates repetition of the coating operation to attain a desired coating film thickness, because a minimum coating film thickness and a minimum hiding property are required to achieve the purpose of protection and coloring of articles with a coating material. In the coating process, excessively rapid evaporation or poor evaporation of the solvent causes pinhole formation in the coating film and sagging of the coating material, tending to give poor appearance of the coating film.
One-component types and two-component types of polyurethane coating materials and adhesives, which employ polyisocyanate of the type of hexamethylene diisocyanate (hereinafter referred to as "HDI") as a curing agent, and an acrylic polyol or polyesterpolyol as the polyhydroxyl compound, are superior in weatherability, chemical resistance, abrasion resistance, etc., and are used in application field of exterior coating and adhesion-bonding in automobiles and buildings. In such polyurethane application field also, the highsolid type system, namely a low viscosity type system, is demanded to decrease the amount of the solvent for dilution to get practical coating viscosity, from the standpoint as mentioned above.
As an HDI type of polyisocyanate curing agent, a polyisocyanurate type one is described to be superior in weatherability in "Polyurethane Resin Handbook", pp. 404-406, (published by Nikkan Kogyo Shimbun Ltd.). However, commercially available curing agents of this type have high viscosity in the state substantially free from HDI monomer and solvent. For example, isocyanurated HDI (Coronate HX, isocyanate content: 21.3%, solid content: 100%, made by Nippon Polyurethane Industry Co.) has a viscosity of 2,400 millipascal.second/measured at 25.degree. C. ("mP.a/25.degree. C.") in the state in which neither an HDI monomer nor a solvent is contained substantially. Even the lowest-viscosity type of isocyanurated HDI (Duranate TPA-100 (isocyanate content: 23.1%, solid content: 100%, made by Asahi Chemical Industry Co., Ltd.) has a viscosity of 1,400 mP.a/25.degree. C.
On the occasion of the study of the main agent and curing agent in order to achieve a high-solid formulation of two-component type polyurethane, the polyhydroxyl compound may be selected from high-solid types, namely low-viscosity or low-molecular weight types, of polyesterpolyol, alkyd polyol, epoxypolyol, acryl polyol and the like. However, in order to obtain a high-solid formulation as a total system, the curing agent also needs to be selected from those having both a lower molecular weight and a lower viscosity than conventional ones. At this time, the curing agent needs to have an isocyanurate ring in the molecule in view of weatherability as mentioned above. However, the low molecular weight type of curing agent itself has a lower limit in molecular weight since it is derived from 3 moles of HDI, so that the viscosity thereof cannot be lower than about 1,400 mP.a/25.degree. C. mentioned above.
For the purpose of solving the above-mentioned problems in conventional polyurethane coating materials and adhesives, the inventors noticed the uretidine dione structure which is of lower molecular weight than the isocyanurate structure, and tried to introduce the uretidine dione structure into the molecule in a large quantity to give a lower molecular weight and a lower viscosity of the curing agent. As the result, the inventors have found a novel HDI type curing agent, and accomplished the present invention.